The present invention relates to a method for applying various solutions onto a continuously running belt-like flexible support web, particularly in the production of photographic sensitive materials such as photographic film, printing paper, or the like.
The following methods have been proposed for increasing the coating speed of a web.
(1) A method in which a high voltage is applied to a coating backup roller so as to prevent a liquid break between a liquid injector and the web (see British Patent No. 1,166,500).
(2) A coating method in which a high DC voltage is applied to a discharge electrode so as to produce a corona discharge to thereby apply unipolar charge to a surface of a web (see U.S. Pat. No. 4,835,004).
(3) A method for coating a web at a higher temperature than room temperature by heating the web before coating (see U.S. Pat. No. 4,835,004).
(4) A coating method in which a high DC voltage is applied to a coating backup roller in the case of coating a wed having a gelatinous undercoat layer containing a surface-active agent (see U.S. Pat. No. 4,837,045).
In the case where a high DC voltage is applied to a coating backup roller as in method (1), or in the case where a high DC voltage is applied to a discharge electrode to produce corona discharge thereby to apply unipolar charges on a surface of a web as in method (2), if the number of polar groups on the surface of the web is relatively large, that is, if a surface-active agent is employed as in method (4), the wetting property of the surface of the web is generally improved to make it possible to increase the speed of coating because of the orientation of the polar groups relative to the surface of the web as well as the electrostatic attraction caused by the applied electric field. However, if the number of polar groups on the surface of the web is relatively small (e.g., not more than 4.times.10.sup.-3 eq/m.sup.2 polar groups per web unit area from the surface of the web to a depth of 1 .mu.m), the wetting property of the surface of the web is not improved due to the above-mentioned orientation of polar groups. Thus, there is a problem in that the speed of coating cannot be increased as much as desired.
Further, method (3) has a problem in that the method cannot be applied to the case where a material, such as polyolefin, having a low melting point and a low glass transition point is used as a material for the web.